Radio frequency (RF) signals are often used in RFID systems in order to communicate with one or more tags, for purposes such as security or tracking. In general, an RFID system uses a transceiver to broadcast an interrogation signal, which is received by any operable RFID tag within the operable range of the transceiver. Upon receiving the interrogation signal, each RFID tag transmits a response signal that encodes unique tag identification information and other stored data. RFID tags can be classified as either passive or active, depending on how they are powered—passive RFID tags are powered via an onboard DC converter that extracts energy from the interrogation signal, and active RFID tags are powered by an onboard battery.
In some applications, simply receiving a response signal is sufficient, as it indicates that the RFID tag (and any person or object associated with the tag) corresponding to the response signal is located within the boundaries defined by the range of the RFID transceiver. Greater granularity may be provided by transmitting each response signal at a known signal strength, given that signal strength decreases in a predictable manner. As such, the distance between the RFID transceiver and a source tag can be approximated by analyzing the signal strength of the response signal received at the transceiver.
RFID systems may be divided into near-field systems and far-field systems, each exhibiting unique properties and requiring a specific type of antenna. The most immediately observable distinction between near-field and far-field is the separation range between tag and transceiver over which the RFID system remains operable. Near-field systems, often found in card readers and other close proximity applications, are constrained to a separation on the order of inches and are polarization agnostic. Far-field systems, often found in asset or product tracking applications, operate at a separation on the order of feet and are polarization sensitive (unable to obtain a reading), particularly at close range. As such, there is a need for a radio frequency system with a single antenna that is operable in both the near-field and the far-field, thereby providing an increased effective range of operation and eliminating the blind spots inherent to each individual mode of operation.